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Mendel and the Gene Idea

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1 Mendel and the Gene Idea
Chapter 14

2 Points to Ponder What is the genotype and the phenotype of an individual? What are the genotypes for a homozygous recessive and dominant individuals and a heterozygote individual? Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and a sex-linked cross). What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and PKU? How are each of the above inherited? What is polygenic inheritance? What is a multifactorial trait? What is sex-linked inheritance? Name 3 X-linked recessive disorders. What is codominance? What is incomplete dominance? What do you think about genetic profiling?

3 These traits are genetically inherited
Genotype and phenotype These traits are genetically inherited Answer these questions about your inheritance: Do you have a widow’s peak? Are your earlobes attached or unattached? Do you have short or long fingers? Do you have freckles? Can you roll your tongue? Do you have Hitchhiker’s thumb?

4 Some genetic vocabulary
Gene: the basic unit of heredity in a living organism. A section of a chromosome that codes for a trait, or characteristic. Alleles: alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes); alleles occur in pairs

5 Genotype and phenotype
Dominant allele: will be expressed and will mask a recessive allele (Tt or TT) Recessive allele: allele that is only expressed when a gene has two of this type of allele Homozygous dominant genotype: 2 dominant alleles (TT or AA) Homozygous recessive genotype: 2 recessive alleles (tt or aa) Heterozygous genotype: one dominant allele and one recessive allele (Tt or Aa)

6 Genotype and phenotype
Genotype – specific genes for a particular trait written with symbols Phenotype – the physical or outward expression of the genotype Genotype Phenotype EE unattached earlobe Ee unattached earlobe ee attached earlobe What are your genotype and phenotype?

7 Understanding genotype & phenotype
Genotype and phenotype Understanding genotype & phenotype Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. egg E e E E e e sperm fertilization zygote EE ee Ee growth and development EE ee Ee unattached earlobe attached earlobe unattached earlobe Allele Key E= unattached earlobes e= attached earlobes

8 Gregor Mendel Documented a particulate mechanism of inheritance through his experiments with garden peas Figure 14.1

9 “particulate” inheritance
Parents pass on discrete heritable units, “genes” (Mendel referred to these as “factors”)

10 Mendel’s experiments Concept 14.1: Mendel used the scientific approach to identify some basic laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned and executed experiments

11 Mendel’s Experimental, Quantitative Approach
Mendel chose to work with peas Because they are available in many varieties Because he could strictly control which plants mated with which

12 Mendel’s experiments : Controls
Wisely, Mendel chose to track only those characters that varied in an “either-or” manner: - dominant or recessive, no blended characteristics, no polygenic traits Mendel also made sure that: He started his experiments with varieties that were “true-breeding” (purebred lines isolated through self-pollination)

13 In a typical breeding experiment
Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are called the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate, the F2 generation is produced

14 ALL of the offspring were purple. Why?
The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants ALL of the offspring were purple. Why?

15 Useful Genetic Vocabulary
An organism that is homozygous for a particular gene Has a pair of identical alleles for that gene (ex: PP or pp) Exhibits true-breeding An organism that is heterozygous for a particular gene (example: Pp) Has a pair of alleles that are different for that gene (hybrid)

16 …but when Mendel crossed the F1 plants:
Many of the F2 plants had purple flowers, but some had white flowers He found a repeatable ratio of about three to one, purple to white flowers, in the F2 generation P Generation (true-breeding parents) Purple flowers White F1 Generation (hybrids) All plants had purple flowers F2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white. Figure 14.3

17 Mendel’s “Factors” (genes)
Mendel reasoned that In the F1 plants, only the purple flower “factor“ was affecting flower color in these hybrids Purple flower color was dominant, and white flower color was recessive In such cases, the dominant allele MASKS the recessive

18 Mendel observed the same pattern in many other pea plant characters 
Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring Four related concepts make up this model… Table 14.1

19 First, alternative versions of genes
Alleles First, alternative versions of genes Account for variations in inherited characters, which are now called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

20 Second, for each character
An organism inherits two alleles, one from each parent A genetic locus is actually represented twice Homologous chromosomes, one paternal, one maternal  Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

21 Dominants and Recessives
Third, if the two alleles at a locus differ Then one, the dominant allele, determines the organism’s appearance The other allele, the recessive allele, has no noticeable effect on the organism’s appearance (is “masked”)

22 Vocabulary review Allele: different forms of a gene (for a trait). Homozygous: purebred; identical set of alleles for a trait Heterozygous: hybrid; nonmatching alleles

23 Segregation principle
Fourth, the law of segregation The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

24 Punnett Squares Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particular genotype.

25 Possible gametes for two traits
20.2 One-and Two-trait inheritance Possible gametes for two traits Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. one pair Allele Key Cell has two pairs of homologues. W = Widow’s peak w = Straight hairline one pair S = Short fingers s = Long fingers either or MEIOSIS I S S s s S S s s W W W W w w w w MEIOSIS II S S s s S S s s W W w w w w W W S S s s S S s s W W w w w w W W WS ws wS Ws

26 Phenotypic ratio approximately 9:3:3:1
A dihybrid cross Illustrates the inheritance of two characters Produces four phenotypes in the F2 generation YYRR P Generation Gametes YR yr yyrr YyRr Hypothesis of dependent assortment independent F2 Generation (predicted offspring) 1⁄2 1 ⁄2 3 ⁄4 1 ⁄4 Sperm Eggs Phenotypic ratio 3:1 Yr yR 9 ⁄16 3 ⁄16 1 ⁄16 YYRr YyRR Yyrr YYrr yyRR yyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F1 Generation RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. Figure 14.8

27 Independent Assortment Principle
Using the information from a dihybrid cross, Mendel developed the law of independent assortment Each pair of alleles segregates independently during gamete formation Animated version:

28 The reality of inheritance
Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely simple What about green eyes? Hazel eyes?

29 Extending Mendelian Genetics for a Single Gene
The inheritance of characters by a single gene May deviate from simple Mendelian patterns Exceptions to the “rules” are listed in the following slides Need practice? Try these “drag and drop” Punnett squares

30 The extent of variation
Not only does meiosis guarantee segregation and independent assortment of alleles, but crossing over also mixes up alleles on homologous chromosomes before distribution “Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, something called crossing over, in which the two homologous chromosomes of a pair exchange equal segments during synapsis in Meiosis I, can add further variation to an individual’s genetic make-up.”

31 Crossing over prior to gamete formation increases variables
Sex and variation Crossing over prior to gamete formation increases variables Independent assortment Crossover animation

32 The Spectrum of Dominance
Complete dominance Occurs when the phenotypes of the heterozygote and dominant homozygote are identical PP pp Pp

33 Incomplete dominance Example: Flower colors RR = red R’R’ = white
The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW White CWCW Pink CRCW Sperm Cw 1⁄2 Eggs CR CR CR CW CW CW Example: Flower colors RR = red R’R’ = white RR’ = pink (intermediate pigmentation) Figure 14.10

34 The Spectrum of Dominance
In codominance Two dominant alleles affect the phenotype in separate, distinguishable ways IAIB = The human blood group type AB is an example of codominance RW = roan coat color in cattle or horses

35 Multiple Alleles and Codominance
Most genes exist in populations In more than two allelic forms Table 14.2 The ABO blood group in humans Is determined by multiple alleles

36 Frequency of Dominant Alleles
Are not necessarily more common in populations than recessive alleles It really depends on whether a trait gives an individual an adaptive advantage, and is naturally selected. Examples: type O blood (ii) recessive present in majority of population

37 Pleiotropy Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is, for example, used by various cells, or has a signaling function on various targets. A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for the enzyme (phenylalanine hydroxylase), which converts the amino acid phenylalanine to tyrosine, another amino acid. Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body. PKU is totally benign if a diet free from phenylalanine is maintained.

38 Which of Mendel’s Laws does this defy? See linkage animation
Gene Linkage Sometimes, when genes are located close together on the same chromosome, they tend to be inherited together. Which of Mendel’s Laws does this defy? See linkage animation A “classic” example of this is why cats with white fur and blue eyes are (more often than not) also deaf.

39 Extending Mendelian Genetics for Two or More Genes
Many traits (especially in complex organisms) May be determined by two or more genes (polygenic) In EPISTASIS “standing upon” the phenomenon where the effects of one gene are modified by one or several other genes (which are sometimes called modifier genes). A gene at one locus alters the phenotypic expression of a gene at a second locus

40 An example of epistasis in mice
Here we see the effect the bb gene combo has on the C gene for the agouti (brownish) fur color in mice cc combo = white fur C_ combo = black fur, unless bb “stands upon” it. Then you get agouti color. BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16 Sperm Eggs Figure 14.11

41 Other variations in gene expression
Incomplete Expressivity is seen in cases where individuals with the same genotypes may have, often for unknown reasons, variability in their phenotypes. This is often seen in genetic diseases where one person with a disease such as diabetes may be very severely effected while another with the same allele may have a milder form of the disease. Incomplete Penetrance is seen when an individual with a particular genotype does not express the phenotype. Again the reasons for this are not clearly understood. For example, known mutations in the gene responsible for Huntington disease have “95% penetrance”, because 5% of those with the dominant allele for Huntington disease don't develop the disease and 95% do.

42 Beyond simple inheritance
Polygenic traits - two or more sets of alleles govern one trait Each dominant allele codes for a product so these effects are additive Results in a continuous variation of phenotypes e.g. skin color ranges from very dark to very light Note that: Environmental effects can cause intervening phenotypes! AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny

43 Polygenic inheritance
Multifactorial trait – a trait that is influenced by both genetic and environmental factors e.g. skin color is influenced by sun exposure e.g. height can be affected by nutrition most are this height Number of Men few few 62 64 66 68 70 72 74 short tall Height in Inches Courtesy University of Connecticut/Peter Morenus, photographer;

44 Genes are affected by Environment
While there is a strong genetic component in human height, the average height has increased over the past 50 years in developed countries. This is considered to be due to improved nutrition. Likewise, a cotton plant may have the alleles necessary for high yields but if it doesn't receive enough water or fertilizer it cannot reach its genetic potential.

45 Example: A phenotypic range of a particular genotype may be influenced by the environment Figure 14.13 Exact color often mirrors the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. This is caused by a color change of the flower pigments in the presence of aluminum ions which can be taken up into the flower.

46 Demonstrating environmental influences on phenotype
Beyond simple inheritance Demonstrating environmental influences on phenotype Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Himalayan rabbit/Siamese cat coat color influenced by temperature There is an allele responsible for melanin production that appears to be active only at lower temperatures The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color © H. Reinhard/Arco Images/ Peter Arnold

47 Humans are not convenient subjects for genetic research. Why?
Many human characters Vary in the population along a continuum and are called “quantitative characters” Humans are not convenient subjects for genetic research. Why? However, the study of human genetics continues to advance

48 A pedigree Is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Ww ww WW or First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ff FF or Ff FF Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

49 Pedigree Analysis Human geneticists illustrate the inheritance of a gene within a family by using a pedigree chart. On such a chart, males are symbolized by a square ( □ ) and females are symbolized by a circle ( ○ ). People who are affected by a disease are symbolized by a dark circle or square. The pedigree chart below shows inheritance of the gene that causes albinism. A and B represent a couple who had five children, including C and E. Only one of the children, E, was albino. E and her husband had five children, including G. In the pedigree below write the genotypes of the individuals who are labeled with letters, using (A) to represent the dominant allele and (a) to represent the recessive allele. Start by indicating the genotypes of E and F. Then use a Punnett Square to figure out what the genotypes for C and D must be. Next, determine the genotypes of A and B. Finally, determine the genotype of G.

50 Types of Gene Mutations
Point mutations: caused by single-base substitution

51 Sickle-Cell Disease Affects one out of 400 African-Americans
Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells (shows seven out of the 146 amino acid units of normal hemoglobin)

52 Electrophoresis of the hemoglobin protein shows:
We see that homozygous normal people have one type of hemoglobin (A) and anemics have type S, which moves more slowly in the electric field. The heterozygotes have both types, A and S. In other words, there is codominance at the molecular level. Try this pedigree of familial inheritance of SC disease

53 Sickle Cell Disease Homozygous recessive individuals have sickle cell disease Heterozygous individuals are normal, but exhibit a mild version of the disease; said to “carry the trait” for SC Homozygous dominant individuals are normal. Genotypes:

54 Sickle Cell Symptoms include physical weakness, pain, organ damage, and even paralysis Sickle-cell disease occurs more commonly in people (or their descendants) from parts of the world such as sub-Saharan Africa, where malaria is or was common, but it also occurs in people of other ethnicities. This is because those with one or two alleles of the sickle cell disease are resistant to malaria since the red blood cells are not conducive to the parasites - in areas where malaria is common there is a survival value in carrying the sickle cell genes.

55 Review: Types of Gene Mutations
Frame shift mutation: caused by insertion or deletion of a base; usually serious because they throw off the codon “reading frame” Result: protein is nonfunctional

56 Recessively Inherited Autosomal Disorders
Many genetic disorders Are inherited in a recessive manner (aa) So, most affected individuals are homozygous (dual inheritance) AA, Aa are normal phenotype, in this case Recessively inherited disorders Show up only in individuals homozygous for the allele Carriers Are heterozygous individuals who carry the recessive allele but are phenotypically normal

57 Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine Lung tissue from a cystic fibrosis patient, showing extensive destruction as a result of obstruction and infection. H2O Cl - nebulizer defective channel percussion vest thick mucus

58 Phenyketonuria PKU is an autosomal recessive genetic disorder that is characterized by an inability of the body to utilize the essential amino acid, phenylalanine. Amino acids are the building blocks for body proteins. In 'classic PKU', the enzyme (phenylalanine hydroxylase), that breaks down phenylalanine is completely or nearly completely deficient. This enzyme normally converts phenylalanine to another amino acid, tyrosine. Without this enzyme, phenylalanine and its' breakdown chemicals from other enzyme routes, accumulate in the blood and body tissues. Infants are born “normal” but if not treated, severe brain problems, such as mental retardation and seizures, will occur. Thus, this disease is GREATLY influenced by environmental factors

59 Other autosomal recessive disorders
Tay-Sachs disease Albinism Hemochromatosis types 1-3: the most common genetic disease in Europe. You can watch this animation:

60 Dominantly Inherited Disorders
Some human disorders Are due to dominant alleles One example is achondroplasia A form of dwarfism that is lethal when homozygous for the dominant allele (DD) Figure 14.15

61 Dominant allele Huntington’s disease
Is a degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of age Lake Maracaibo, Venezuela Village pedigree

62 X-linked disorders Sex-linked inheritance
More often found in males than females because recessive alleles are always expressed Most X-linked disorders are recessive: Color blindness: most often characterized by red-green color blindness Duchenne’s muscular dystrophy (DMD): characterized by wasting of muscles and death by age 20 Fragile X syndrome: most common cause of inherited mental impairment Hemophilia: characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all

63 X-linked disorders Sex-linked inheritance X B Y b Key
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. X B Y b Key =Unaffected female =Carrier female =Color-blind female =Unaffected male =Color-blind male grandfather daughter grandson X-linked Recessive Disorders • More males than females are affected. • An affected son can have parents who have the normal phenotype. • For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. • The characteristic often skips a generation from the grandfather to the grandson. • If a woman has the characteristic, all of her sons will have it.

64 Mating of Close Relatives
Matings between relatives are called “consanguineous” matings Can increase the probability of the appearance of a genetic disease (especially harmful recessive homozygotes)

65 X-linked disorders: Hemophilia
Sex-linked inheritance X-linked disorders: Hemophilia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Key Unaffected male Hemophiliac Unaffected female Carrier Queen Victoria Prince Albert 4 children of 9 are shown V ictoria Frederick III (Germany) Alice Louis IV (Hesse) Princess Helena of Waldeck Leopold (died at 31) Beatrice Prince Henry of Battenberg 12 children of 26 are shown Henry Irene Frederick (died at 3) Alexandra Nicholas II (Russia) Alice Alexander (Earl of Athlone) Alfonso XII (Spain) Victoria Leopold (died at 32) 6 children of 34 are shown Waldemar (died at 56) Henry (died at 4) Alexei (murdered) Rupert (died at 21) Alfonso (died at 31) Gonzalo (died at 20) (queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images

66 Multifactorial Disorders
Many human diseases Have both genetic and environment components Examples include Heart disease and cancer Try this interactive family pedigree (nicotine addiction)

67 Genetic Testing and Counseling
Genetic counselors Can provide information to prospective parents concerned about a family history for a specific disease

68 Counseling Based on Mendelian Genetics and Probability Rules
Using family histories Genetic counselors help couples determine the odds that their children will have genetic disorders

69 Tests for Identifying Carriers
For a growing number of diseases Tests are available that identify carriers and help define the odds more accurately Fetal Testing In amniocentesis The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) A sample of the placenta is removed and tested

70 Prenatal Genetic Screening
Genetic Screening (karyotyping) is possible, prenatally Chorionic villus sampling 9-14 weeks weeks

71 Concept: Some inheritance patterns are exceptions to the standard chromosome theory
Epigenetics Functional modifications to the genome that do not involve a change in the nucleotide sequence. Environmental factors can alter the way our genes are expressed, making even identical twins different. Examples of such modifications are DNA methylation and histone modification, both of which serve to regulate gene expression without altering the underlying DNA sequence.

72 Genomic Imprinting What Is Imprinting?
For most genes, we inherit two working copies -- one from mom and one from dad. But with imprinted genes, we inherit only one working copy. Depending on the gene, either the copy from mom or the copy from dad is epigenetically silenced. Silencing usually happens through the addition of methyl groups during egg or sperm formation. The epigenetic tags on imprinted genes usually stay put for the life of the organism. But they are reset during egg and sperm formation. Regardless of whether they came from mom or dad, certain genes are always silenced in the egg, and others are always silenced in the sperm.

73 (a) A wild-type mouse is homozygous for the normal igf2 allele.
Genomic imprinting Involves the silencing of certain genes that are “stamped” with an imprint during gamete production In mammals the phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father (a) A wild-type mouse is homozygous for the normal igf2 allele. Normal Igf2 allele (expressed) with imprint (not expressed) Paternal chromosome Maternal Wild-type mouse (normal size) Mutant lgf2 allele Dwarf mouse Normal size mouse When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.

74 Genes located outside the nucleus also have influence
Extranuclear genes Are genes found in organelles in the cytoplasm The inheritance of traits controlled by genes present in the chloroplasts (ctDNA) or mitochondria (mtDNA) Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg Figure 15.18

75 In Humans: Some diseases affecting the muscular and nervous systems are caused by defects in mitochondrial genes that prevent cells from making enough ATP Examples of mitochondrial DNA diseases: Leber's hereditary optic atrophy - causes progressive visual impairment Kearns-Sayre disease Progressive external ophthalmoplegia Myoclonus epilepsy MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) Mitomap - A human mitochondrial genome database

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